1. Field of the Invention
The present invention relates to the floor panel structure of a car body and particularly to the floor panel structure of a car body where the floor of an automobile comprises floor panel provided connected to frame members of the car body.
2. Conventional Art
Vibration from frame members linked to the engine or suspension is known to be transmitted to floor panels, causing the floor panels to vibrate and as a result, the air within the passenger cabin vibrates greatly, thus generating unpleasant in-cabin vibrations and noise.
In this case, the source of vibration causing the problem may be vibration from the engine itself or road noise transmitted from the suspension, while this road noise typically includes components due to resonance of the tire cavity and components due to resonance of the suspension.
Typical measures conventionally taken to suppress this vibration and noise include applying vibration-damping materials and vibration-suppressing materials as various vibration-damping and vibration suppression measures. While it is possible to reduce vibration and noise in this manner, an extremely large amount of vibration-damping material and vibration-suppressing material is required, thus increasing the vehicle weight and leading to various deleterious effects and becoming a major problem on the cost side.
Moreover, the unpleasant vibration transmitted from the engine and suspension is mainly equal to or below 400 Hz in an automobile, and in particular, has a peak at a frequency near the 250 Hz which is road noise arising from tire cavity resonance. Thus, a technique is known by which a plurality of beads is formed in the floor panels, thus increasing the panel thickness and raising its rigidity, thereby shifting the natural frequency of the floor panel to a high band higher than 400 Hz. Specifically, an attempt is made to prevent the floor panel from resonating at the resonance frequency of the suspension and the tire cavity resonance frequency band, thus reducing unpleasant vibration and noise.
In this case, while this has the advantage of being able to suppress resonance peaks in low-frequency regions, vibration in the high-pitched regions conversely increases, so it becomes necessary to use large amounts of vibration-damping materials and vibration-suppressing materials in order to suppress vibration and noise in the high-frequency regions. In this manner, even in this case, the vehicle weight is increased as described above so there are various deleterious effects and problems on the cost side, so it is desirable to solve this problem.
Thus, the present inventors focused on the relationship between the vibration frequencies and vibration modes of vibrations transmitted to the floor panel and proposed a structure of a floor panel that has a vibration mode adjusting structure wherein the acoustic emission levels at specific vibration frequencies (resonance regions) become even smaller vibration modes as shown in Japanese Patent Unexamined Publication No. 9-202269 (JP-A-9-202269). This floor panel structure is one wherein the specific frequencies are frequencies near the 250 Hz of road noise arising from the tire cavity resonance transmitted to the floor panel as the most unpleasant vibration, and so the rigidity of the floor panel is partially adjusted so that the vibration mode of the floor panel becomes a vibration mode such as a 2×2 mode or 2×1 mode where an even number of vibration antinodes is generated, and thus with a setup where the sound waves radiated from the respective vibration antinodes cancel each other, it is possible to reduce the acoustic emission level and reduce noise within the cabin.
However, in the conventional case in which vibration-damping materials and vibration-suppressing materials are attached to the entire surface of the floor panel as described above, there are problems of increased materials costs and increased vehicle weight. In addition, if the panel thickness is increased, there is also a problem of increased vehicle weight.
In addition, with the floor panel structure recited in the Japanese Patent Unexamined Publication described above, in order to generate vibrations in a 2×1 mode, for example, the vibration region of the floor panel must have a roughly 2×1 rectangular shape, but because of the placement of components present below the floor of the automobile including the driveshaft, differential and other drive train components, suspension and other suspension system components, exhaust pipes, mufflers and other exhaust system components and the gasoline tank and the like, and also because of the relationship with the seat layout within the cabin, there are limitations to the layout of frame members, so there are cases wherein the floor panels attached to the floor members cannot be given a roughly 2×1 rectangular shape, and thus there is a problem in that it is not possible to generate vibrations in a 2×1 mode in this case.
On the other hand, when the vibration region of the floor panel is not a roughly 2×1 rectangular shape, it may be conceivable to make a 2×1 vibration region by using a highly rigid bead or the like to form a rectangular-shaped vibration region, but if the panel rigidity of the floor panel is high or the vibration region is relatively narrow, then the resonance frequency that the floor panel itself originally has is high, and if such a bead is provided on such a floor panel, then the rigidity of the floor panel is greatly increased and the resonance frequency is increased even further, resulting in the vibrations of the 2×1 mode being generated in a frequency band higher than 250 Hz, for example, and thus there is a problem in that it is not possible to generate 2×1 modes in frequencies near the 250 Hz which is the road noise arising from the cavity resonance of tires in particular.
In addition, even if 2×1 mode vibrations are generated in a vibration region that is not a 2×1 rectangular shape, because of differences in the distribution and amplitudes of the two antinodes of vibration in that 2×1 mode, a problem arises wherein the respective vibration volumes of the two antinodes become different and the effect of the sound waves radiated from the antinodes of the respective vibration canceling each other out becomes extremely small.
Here, the present inventors took note of the relationship between the rigidity of the floor panel and the vibration modes and thus attempted to solve the aforementioned problems with conventional art.